net: decnet: dn_table: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 115106
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/decnet/dn_table.c

@@ -156,6 +156,7 @@ static void dn_rehash_zone(struct dn_zone *dz)
 	default:
 		printk(KERN_DEBUG "DECnet: dn_rehash_zone: BUG! %d\n",
 		       old_divisor);
+		/* fall through */
 	case 256:
 		new_divisor = 1024;
 		new_hashmask = 0x3FF;